内燃机非平衡湍流条件下边界层瞬态流动及其对壁面传热过程影响研究

Near-wall turbulent boundary layer plays a decisive impact on the wall heat transfer in internal combustion (IC) engine, and ultimately affects the combustion, pollutant emission and thermal efficiency. The boundary layer flow in internal combustion engine is significantly different from the steady turbulent boundary layer, which is influenced by the non-equilibrium turbulence on account of the piston high speed reciprocating motion and the valves periodic opening and closing. But there is no clear understanding of the mechanism of the non-equilibrium turbulence on the boundary layer. In the present study, by using stereoscopic particle image velocimetry (stereo-PIV) and micro-PIV technology in the optical engine, the in-cylinder bulk flow and the boundary layer flow will be measured with the purpose of clarifying the non-equilibrium characteristics of turbulence in the cylinder and investigating the mechanism of the non-equilibrium turbulence on the transient boundary layer systematically. A non-equilibrium wall model, which can capture the flow and heat transfer of the boundary layer under complex turbulent conditions of IC engine, will be constructed to reveal the mechanism of the effects of the transient boundary layer flow on the wall heat transfer. The present study will deepen the understanding of the boundary layer flow and wall heat transfer characteristics under non-equilibrium turbulence, and improve the theory of boundary layer flow and heat transfer of IC engine. And the research results will provide theoretical basis for the combustion control of new fuels and new combustion modes, and provide guidance for the control of combustion process and emission in the IC engine, and the development of the high thermal efficiency combustion system.

近壁湍流边界层直接决定着内燃机壁面传热特性，并最终影响内燃机的燃烧、污染物排放和热效率。内燃机内边界层流动受非平衡态湍流(活塞高速往复运动和气门周期性开闭所致)的影响，明显异于稳态湍流边界层，然而非平衡湍流对边界层的作用机制尚不明确。本项目拟采用高速stereo-PIV和micro-PIV方法，在光学发动机上对内燃机缸内宏观流动进行三维高速测量，深入分析缸内湍流的非平衡特性；对边界层进行微观测量，系统研究缸内边界层的瞬态特性和非平衡湍流对边界层流动的作用机理；构建能够捕捉内燃机复杂湍流条件下边界层流动与传热过程的非平衡壁面模型，揭示瞬态边界层流动对壁面传热的影响机制。本研究将深化人们对内燃机非平衡湍流条件下边界层流动与壁面传热特性的认识，完善内燃机边界层流动和传热理论；为新燃料和新型燃烧方式下的燃烧控制提供理论依据，并为进一步提高内燃机热效率，降低污染物排放提供有益指导。

内燃机缸内湍流受活塞往复运动影响，时刻处于非平衡状态，导致内燃机近壁湍流边界层流动明显异于稳态湍流边界层。深入认识非平衡湍流条件下的边界层流动及传热过程是内燃机热效率提升和燃烧控制的重要理论基础。.本项目首先研究了内燃机缸内湍流场的各向异性、非平衡特性及其影响因素，分析了湍流涡团(拟序结构)的瞬态演化过程。研究发现，在大尺度滚流作用下，缸内湍流场各向异性明显，并具备轴对称湍流特征。缸内湍流的非平衡状态不仅决定于发动机转速，同时受缸内大尺度滚流影响较大，本研究设定实验条件下缸内湍流非平衡度(湍流特征时间尺度与平均流变形时间尺度之比)在10至20 之间。缸内湍流场涡团结构在不同循环演变过程有较大差异，其中拟序结构变动是导致发动机缸内流场循环变动的首要因素。在此基础上，本项目研究了内燃机缸内湍流边界层流动特性，揭示了缸内湍流边界层与流场核心区之间的相互作用机制。研究表明，内燃机缸内边界层内粘性底层流动规律与稳态湍流边界层壁面律吻合良好，但是在对数层，受发动机转速以及缸内大尺度流动(滚流)强弱影响很大，难以用统一的壁面律来描述。最后，本项目研究了湍流速度边界层对热边界层内湍流传热过程的影响以及能量传输过程。研究发现，内燃机缸内湍流热边界层内层无量纲温度分布与稳态湍流热边界层壁面律吻合较好，但外层难以用统一的壁面律来表征。湍流传热在缸内壁面对流传热过程中占据主导地位，壁面法向脉动速度直接影响近壁湍流传热量，湍流传热热流密度在近壁区域会出现峰值。